In order to have a better understanding of the present invention, description will first be made of a conventional battery charging device, with reference to FIG. 1, which includes a DC power source 1, constant-current control circuit 2, constant-voltage control circuit 3, current detecting circuit 4, and switch 6 for detecting whether or not a battery 5 is connected in position. With this conventional device, by turning on a charging-starter switch 7, DC power is obtained at output terminals 8A and 8B so that the battery 5 connected therewith can be charged.
FIG. 3 illustrates the relationship between the output voltage V and output current I which occur at the output terminals 8A and 8B. As will be noted, the relationship represents a so-called drooping characteristic. Thus, if the battery is connected after the charging-starter switch 7 has been turned on, then a constant-voltage condition in which there prevails a charging voltage V.sub.1 which is higher than a voltage V.sub.3 prevailing at the end of the charging operation, will be changed to a constant-current condition where a charging current I.sub.l flows. In FIG. 3, V.sub.2 represents a stand-by voltage which prevails after the battery 5 has been charged up or when the battery 5 is not connected. The stand-by voltage V.sub.2, which is used primarily for preventing occurrence of an overcurrent, is selected to be lower than the voltage V.sub.3.
Whether or not the battery 5 is connected in position, is detected by means of the switch 6 which is provided in the neighborhood of the output terminals 8A and 8B and arranged to be pressed by the battery 5 when the latter is connected in position. Completion of the charging of the battery 5 can be detected by using a timer (not shown) and/or means for detecting the peak voltage of the battery.
However, the above-described conventional device is disadvantageous in that the position where the switch 6, which is a mechanically-structured component, can be provided is limited to a position where it can be disposed into contact with the battery 5, thus deteriorating the space efficiency in the arrangement of the entire components. Obviously, this is undesirable from the standpoint of miniaturization of the battery charging device.